A conventional navigation apparatus detects the present position of a vehicle by using a GPS or the like while the vehicle is traveling, displays the present position over a road map on a display unit, and sets a proper route from the present position to a destination as guidance toward the destination. Such a navigation apparatus contributes to easier driving. It is to be noted that, as driving guidance, the proper route from the present position to the destination is shown in a color different from other roads on the map on the display unit or shown by a thick line on the display unit. Besides such a proper route, the driving guidance also enlarges the display of an intersection, which the vehicle enters next, and outputs guiding sounds in order to lead the driver to the destination.
In addition, in a process to set the proper route from the present position to the destination, in general, Dijkstra's method or a technique conforming to the method is adopted. More specifically, the cost (or the evaluation value) of each route from the present position to the destination is computed by using information on links between nodes. When the calculation of the cost of every link from the present position to the destination is completed, links providing a minimum total cost are connected to each other to set a route from the present position to the destination. In a process to set a route from the present position to the destination, however, a road on which the vehicle is not allowed to travel is not included in the route. Examples of such roads are a one-way road or a road specially provided for pedestrians. If information on link interconnection reveals such vehicle-travel restrictions, the route from the present position to the destination can be set by exclusion of roads each subjected to the restrictions.
The vehicle-traveling restrictions established for roads such as a one-way road and a road specially provided for pedestrians do not necessarily apply to all vehicles uniformly. As an example, one of road systems seen in freeways in big cities mainly in the U.S. includes a HOV lane (carpool lane). A carpool lane is provided to recommend car sharing for the purpose of reducing the number of vehicles traveling on roads. Specifically, only a high occupancy vehicle (HOV) having a plurality of passengers therein is allowed to travel along a carpool lane. When the driver of a vehicle is the only passenger in the vehicle, the vehicle is subjected to the vehicle-traveling restriction established for the carpool lane. In the U.S., there are relatively many vehicles not having passengers other than the driver. Thus, in comparison with ordinary roadways, the traffic of a carpool lane is light. As a result, a vehicle with one or more passengers besides the driver is allowed to travel along a carpool lane and hence capable of substantially shortening the travel time.
In some cases, a carpool lane is provided separately from an ordinary roadway, which does not require that a traveling vehicle have at least one passenger besides the driver of the vehicle. However, such a carpool lane may coexist with an ordinary roadway on the same road as two lanes having the same traveling direction. That is, the road is divided into lanes by one or more lines, or the lanes are merely partitioned from each other by a partitioning line (or a lane mark). In some cases, the carpool lane and the ordinary roadway are partitioned from each other by a guard rail. In this case, a vehicle is allowed to change its traveling path from the carpool lane to the ordinary roadway or vice versa only in a predetermined traveling-path changing area. That is, a vehicle is allowed to leave the carpool lane and enter the ordinary roadway or vice versa only in the predetermined traveling-path changing area.
On the other hand, an exit from a freeway including a carpool lane and an ordinary roadway to another road is provided and available only to the ordinary roadway of the freeway. By viewing a display appearing on a navigation apparatus as the display of a map showing the vicinity of a vehicle or by using a route navigation function, the driver of a vehicle traveling along a carpool lane may be aware of the fact that the vehicle is approaching such an exit of the freeway. Nevertheless, the vehicle is not capable of entering the ordinary roadway from the carpool lane. This is because there is no point to depart from the carpool lane and enter the ordinary roadway or there is no point of changing a traveling path from the carpool lane to the ordinary roadway. As a result, in some cases, the vehicle inevitably travels past the exit of the freeway. In particular, in the case of a freeway provided in a big city as a freeway having a number of ordinary roadways, this problem arises more often.
In order to solve the above problem, a vehicle traveling along a carpool lane should return to an ordinary roadway in well advance of taking an exit. When there are several points to depart from the carpool lane and enter the ordinary lane prior to such an exit of the freeway and the vehicle return to the ordinary roadway through an early departure point, the distance of traveling along the carpool lane unavoidably becomes short, raising another problem of not being capable of traveling with a high degree of efficiency. This problem is raised not only in a vehicle traveling along a carpool lane, but the problem is also raised in a vehicle traveling on an ordinary roadway subjected to a restriction on the traveling direction.
A solution addressing such a problem is disclosed in U.S. Pat. No. 6,804,604 (JP 2001-183159A). In accordance with this solution, by including road information provided as map data and carrying out navigation processing by using the map data, a vehicle is capable of traveling along a carpool lane with a high degree of efficiency. This is because a navigation operation is carried out by considering restrictions on changing a traveling path from the carpool lane to an ordinary roadway and vice versa or restrictions on departing from the carpool lane to enter an ordinary roadway and vice versa.
Even when navigation processing is carried out by using navigation data created by taking carpool lanes into consideration, the solution still has the following problems.
(1) The solution assumes that the present position is on a carpool lane without determining whether the present position is on a carpool lane or an ordinary roadway. Thus, when a traveling-path changing area, which allows a vehicle to change its traveling path from a carpool lane to an ordinary roadway, exists ahead of the vehicle, the driver of the vehicle is merely informed of the existence of the traveling-path changing area. Accordingly, when the vehicle driver notified of the existence of the traveling-path changing area is capable of correctly determining whether the present position is on a carpool lane or an ordinary roadway, the notice of the existence of the traveling-path changing area may be useful to the driver in its own right. It is very likely, however, that the driver of the vehicle is not capable of correctly determining whether the present position is on a carpool lane or an ordinary roadway. For a driver not capable of correctly determining whether the present position is on a carpool lane or an ordinary roadway, a navigation apparatus also capable of providing user-friendly guidance is preferable.
(2) For example, a vehicle has departed from a carpool lane of a freeway to enter an ordinary roadway of the same freeway in an attempt to leave the freeway through an exit of the freeway to travel on another road. In this case, it is possible to consider that there is no necessity to inform the driver of the existence of the traveling-path changing area allowing the vehicle to change its traveling path from a carpool lane to an ordinary roadway thereafter or, when there is such necessity, the degree of the necessity is considered to be extremely low. In accordance with the above solution, however, when a traveling-path changing area allowing a vehicle to change its traveling path from a carpool lane to an ordinary roadway exists ahead of the vehicle, the driver of the vehicle is notified of the existence of the traveling-path changing area. Thus, for the driver of a vehicle, the notice of the existence of the traveling-path changing area is not required or, when required, the notice will be guidance of a low level of necessity.
(3) It is difficult for a position detection unit using the GPS to determine a lane, on which the present position of a vehicle is located, with a high degree of precision. A communication between an onboard device mounted on the vehicle and on-road devices provided on a road can be carried out to determine the type of a lane, on which the present position of a vehicle is located. However, this solution to the problem is based on the assumption of complete infrastructure equipment including the on-road devices provided on a road. Thus, from the practical point of view, this solution to the problem is difficult to implement.
Real implementation of a carpool lane has been taken into consideration so far. However, the same problem is also raised in a road structure wherein ‘a plurality of roads having the same traveling direction has a configuration including a special roadway having no exits to other roads and an ordinary roadway having exits to other roads, and a vehicle is allowed to change its traveling path from the special roadway to the ordinary roadway or vice versa only in a predetermined traveling-path changing area.’ Examples of the special and ordinary roadways are respectively an express lane and a local lane, which exist in the East-Coast areas of the U.S.